Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is employed to plug and seal off undesirable flow passages in the cement sheath and/or the casing. While a cement slurry is one type of sealant composition used in primary and secondary cementing operations, other non-cement containing sealant compositions may also be employed.
Latex emulsions, which contain a stable water-insoluble, polymeric colloidal suspension in an aqueous solution, are commonly used in sealant compositions to improve the properties of those compositions. For example, latex emulsions are used in cement compositions to reduce the loss of fluid therefrom as the compositions are being pumped to the annulus. Latex emulsions are also employed to reduce the brittleness of the sealant compositions; otherwise the compositions may shatter under the impacts and shocks generated by drilling and other well operations. Such sealant compositions may be used for sealing the junction of multilateral wells. In addition, latex emulsions are used to improve the flexibility of sealant compositions.
Additionally, latex emulsions are utilized to prevent gas migration during a transition phase in which the sealant composition changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. When first placed in the annulus, the sealant composition acts as a true liquid and thus transmits hydrostatic pressure. During the transition phase, certain events occur that cause the sealant composition to lose its ability to transmit hydrostatic pressure such as the development of a solid (i.e., stiff) structure in the composition. When the pressure exerted on the formation by the sealant composition falls below the pressure of the gas in the formation, the gas initially migrates into and through the composition. The gas migration causes flow channels to form in the sealant composition, and those flow channels permit further migration of the gas after the sealant composition sets.
Moreover, latex emulsions are also mixed with drilling fluids, particularly the non-aqueous type, near loss-circulation zones such as natural or induced fractures, thereby forming solid masses for sealing those zones to prevent the drilling fluids from being lost during drilling.
Traditional latex emulsions prepared by emulsion polymerization usually become unstable in the presence of salt. That is, the polymer particles contained in the latex typically fall out of the aqueous solution and form a separate rubbery phase when exposed to salt. Unfortunately, sealant compositions often come into contact with salts that are naturally present in the wellbore. Further, the sealant compositions themselves often contain salts of monovalent, divalent, and occasionally trivalent cations. They may even be saturated with such salts to ensure that they do not wash out or dissolve salt zones located in the subterranean formation. To improve the tolerance of latex emulsions in sealant compositions to salts, especially those containing monovalent and divalent cations, surfactants such as ethoxylated nonylphenol sulfates are included in the compositions. The use of such surfactants in the sealant compositions undesirably increases the overall cost of constructing and maintaining the wellbore. A need therefore exists to use latexes in sealant compositions, such as cement slurries, that are stable in the presence of salts.